Japanese Patent Nos. 2,543,269 and 2,543,270 both registered on Jul. 25, 1996 disclose techniques which may be used for welding such workpieces as described above. According to these Japanese patents, a power supply is connected to a welder electrode and a workpiece for supplying current to them. One cycle of the current includes an AC current period during which an AC current is supplied and a DC current period following the AC current period, and the cycle is repeated. During the DC current period, a DC current flows from the workpiece to the welder electrode. According to Pat. No. 2,543,269, the proportion of the AC current period in one cycle period, i.e. the AC current period plus the DC current period, is from 30% to 80%.
According to the techniques disclosed in these Japanese patents, the surface of a workpiece of aluminum, for example, is purified of an oxide film covering the surface during the AC current period and, particularly, during part of the AC current period in which the workpiece is at a negative potential. Also, according to these techniques, since the polarity of the welding current is not inverted during the DC current period, during which the workpiece is at a positive potential, no noise which would otherwise be generated by high harmonics is generated, resulting in reduction of arcing sound.
Sometimes, the proportion of the DC current period in one cycle is increased to a value greater than from 70% to 20% of one cycle in order to effect deep welding of the workpiece. In this case, the workpiece is welded by DC arcing before the surface of the workpiece is sufficiently purified of the oxide film thereon because of shortened AC current period. As a result, slag may be introduced into the melt of the workpiece, which may produce defects in the welded portion.
An object of the present invention is to provide a welding method which can effect welding with reduced welding defects, and also a power supply apparatus for use in practicing such welding method.